Why can't I plug in vertex normals to rotation of Object Transforms Output
Vertex normals are vectors (transparent blue) and Rotation of object is in 'Euler' format (transparent purple). So, you have to first convert Vertex Normals Vectors to Euler then you can plug into the rotation socket. There are two nodes in animation nodes which can do this, 'Vector to Euler' node and 'Direction to rotation' node. However, 'Direction to rotation' node has many controls so you can choose 'guide axis (here is 'x')' and 'track axis (here is 'z')'.

Vector to Euler Node


Direction to Rotation Node



:)